1. Field of the Invention
This invention relates to a process for using oxygen (industrially pure oxygen, and this is hereunder referred to as oxygen) to refine pig iron in a converter or a like refining vessel. More particularly, it relates to a process for refining pig iron by supplying oxygen from above the melt together with a gas such as oxygen or a mixture of oxygen and a slow-reactive gas which is supplied from the bottom of the melt through sheath nozzles.
2. Description of the Prior Art
A steel making process that blows pure oxygen onto the surface of molten metal in a converter is conventionally known as "LD" process. When the carbon content of the melt is high, the energy produced by the impact of blown oxygen and the stirring action of carbon monoxide generated in the melt cause active refining of the iron, but when the carbon content is reduced to less than 0.8 wt%, particularly to a level close to 0.1 wt%, the formation of carbon monoxide becomes slow whereas the force of stirring the molten steel bath is weakened and the decarburizing rate is reduced. In consequence, the oxygen content in the molten steel increases rapidly to provide excess oxygen. This increases the content of iron oxides in the slag, and as a result, a sizable amount of iron and manganese is lost from the melt at the end point of the refining operation, and what is more, the yield of addition of alloy elements such as Mn, Si and Al to the molten steel is decreased. These problems have been the cause of a considerable economic loss in the LD process.
West German Pat. No. 1909779 teaches a process for refining pig iron by using sheath nozzles (comprising two coaxial pipes) which was already disclosed in French Pat. No. 1450718, and this process is characterized by supplying the iron melt with both oxygen and lime powder from beneath the converter through the inner pipe of the sheath nozzles. Hydrocarbon is supplied through the annular space between the inner and outer pipes as a coolant gas. This proposal has enabled the use of oxygen instead of air that has been employed in a Thomas converter which is the existing bottom-blown converter. It also retains the reasonable life of the converter by protecting the furnace bottom lining and sheath nozzles with the coolant gas. Therefore, the proposal has been put to commercial use under the name "OBM/Q-BOP" process.
However, even this process has the following defects. When the carbon content of the hot metal decreases and the production of carbon monoxide slows down, the hydrogen content of the molten steel increases to 4 to 6 ppm. The increased amount of hydrogen causes one problem or another in the step subsequent to the refining operation, and a certain type of steel will require dehydrogenation. What is more, since a large amount of oxygen which is very active and causes a vigorous and explosive reaction is blown from the bottom of the converter through sheath nozzles, the stirring of the melt has a tendency to go excessively. For these reasons, the process finds difficulty in slag formation of lime that is added as flux material for refining, and there is considerable slopping (the overflowing of slag and molten steel) and sticking metal skulls to the walls of the furnace mouth. Slopping can only be made less vigorous by supplying lime powder with an oxygen jet and suppressing the explosive reaction of oxygen, but to blow lime powder from the bottom of the furnace, additional sheath nozzles and hence more hydrocarbon for cooling them become necessary. This is the cause of the production of a low-carbon steel with high hydrogen content and presents other problems with equipment and maintenance that include considerable erosion of the bottom lining,, production and transport of lime powder, the technique for providing even distribution of lime powder through a plurality of nozzles, and protection against the wear of oxygen blowing pipes by lime powder. As a further disadvantage of the OBM/Q-BOP process, dephosphorization does not proceed to a satisfactory level with a high-carbon steel containing more than 0.25 wt% of carbon, and therefore, carburization becomes necessary wherein the carbon content of the steel melt must first be reduced to less than 0.10 wt% to achieve desired dephosphorization and then a large amount of a carburizing material is added to the melt being tapped.
U.S. Pat. No. 3,953,199 proposes a method which it claims eliminates the defects of the LD process and OBM/Q-BOP process. The method is basically the combination of top blowing and bottom blowing of oxygen wherein pure oxygen is blown onto the surface of the melt through a lance and at the same time pure oxygen is also blown from the bottom of the furnace through a sheath nozzle. What is unique about this method is that in the early period of refining operation, refining is substantially achieved by oxygen blown from above and when the efficiency of above blown oxygen for decarbonzation reaction begins to decrease, the oxygen supply from below is increased immediately and refining is substantially achieved by oxygen supplied from the sheath nozzle. According to the illustrated embodiment of this proposal, since lime can be added together with oxygen being supplied from above, the temperature of the slag increases to promote slag formation. In the last stage of refining by this process the carbon content of the melt is low, the production of carbon monoxide is little and the stirring of the melt is weak. In such stage, the flow rate of oxygen supplied from the sheath nozzle must be increased to about 50%, and thus, even if the flow rate of oxygen blown from below in the early and intermediate stages of refining is held to minimum level that can prevent the melt from entering the sheath nozzle, a considerable amount of oxygen is blown from below in all. Therefore, the proposed method blows a large volume of oxygen into the melt from the sheath nozzle, presenting the same problems encountered with the OBM/Q-BOP process, i.e. difficulty in forming a slag from lime, slopping, and sticking of metal skulls to the walls of the furnace mouth. To solve these problems, the process, as taught in the embodiment shown, blows a mixture of lime powder and oxygen onto the melt surface and achieves the same effect as obtained by the OBM/Q-BOP process that blows lime from below. What is more, the U.S. patent described the effect and advantage of the proposed process on a pure qualitative basis and therefore one cannot determine whether it is truly effective.
Belgian Pat. No. 780910 also describes a process that combines top blowing and bottom blowing, but its primary object is to increase thermal efficiency by using top-blown oxygen to burn the carbon monoxide generated upon reaction with bottom-blown oxygen. Therefore, it incorporates a technical concept that entirely differs from this invention which, as will be described hereunder, has for its primary object a great improvement in the refining capability of top-blown oxygen.
The refining process proposed by Belgian Pat. No. 872620 aims at increasing the thermal efficiency of a converter and increasing the charge of scrap by blowing oxygen from above as well as from below. According to this process, 20 to 80% of the total oxygen is blown on to the melt surface through nozzles installed on the side walls in the upper part of the converter and the remaining part of the oxygen is supplied from nozzles in the bottom together with lime powder. The process greatly differs from this invention with respect to the amount of oxygen to be blown from the bottom nozzle. Another difference is that according to the process of the Belgian patent, satisfactory refining is difficult without supplying powder from the bottom sheath nozzle. To be more specific, our invention limits the flow rate of bottom-blown oxygen to 2 vol% to 17 vol%, preferably from 2 vol% to 13 vol%, thereby implementing the supply of lime blocks from the furnace mouth as has been effected in the conventional top-blowing converter instead of using the complicated means of blowing lime powder from above or blowing it from below together with oxygen. This invention is capable of producing a steel whose hydrogen content is not much different from that of the steel made by the conventional top-blowing converter and it can be implemented with simpler equipment. As a further advantage, the invention maintains high refining efficiency while it assures constant lancing conditions. Such advantages of this invention cannot be expected from the processes of prior patents.